The arc discharge tube of a conventional high pressure discharge lamp contains a metal which is vaporized in the plasma arc during lamp operation. In particular, in a high pressure sodium lamp, for example, it is well-known that the self-absorption characteristics of sodium atoms tend to limit lamp efficacy. That is, cooler sodium atoms existing proximate the walls of the arc discharge tube absorb some of the visible sodium D-line radiation produced within the hot central plasma region of the arc tube. One solution to this problem is described in application Ser. No. 454,225 of P. D. Johnson filed Dec. 29, 1982, now abandoned. According to this prior patent application, a predetermined amount of iodine is added to the plasma. In this way, while substantially all the sodium iodide in the central region of the plasma arc is dissociated, none of the sodium iodide near the cooler arc tube wall is dissociated. Since sodium iodide cannot absorb radiation from sodium atoms, lamp efficacy is increased. Moreover, commonly assigned U.S. Pat. No. 4,605,881, issued to J. T. Dakin on Aug. 12, 1986, teaches the addition of iodine to the discharge tube in excess of sodium iodide stoichiometry (i.e., all sodium and iodine combined) in order to eliminate free sodium near the arc tube walls when xenon is employed as a buffer gas instead of mercury vapor, which had been conventionally used, thus further improving efficacy.
Disadvantageously, however, efficacy in the aforementioned lamps is still limited in conventional long, thin arc tubes by the "de-mixing" of sodium and iodine. That is, in long, thin arc tubes, sodium remains preferentially near the arc tube walls, and iodine remains preferentially in the central region of the arc tube. A solution to the "demixing" problem would be to employ short, wide arc tubes which promote convective mixing of the sodium and iodine. However, electrode losses are a higher percentage of the total input power in a short, wide arc tube. Hence, there are competing factors in the optimization of electroded HID lamps.
These competing factors in the optimization of electroded HID lamps have led to the development of electrodeless HID lamps. An electrodeless HID lamp comprises an arc tube for containing a fill and an excitation coil coupled to a radio frequency power source; the excitation coil surrounds the arc tube for exciting a plasma arc discharge in the fill. One such lamp is disclosed in U.S. Pat. No. 4,783,615, issued to J. T. Dakin on Nov. 8, 1988 and assigned to the assignee of the present invention. This patent, which is hereby incorporated by reference, teaches the use of xenon as a buffer gas in an electrodeless sodium iodide HID lamp. Very high efficacies are achieved by using an arc tube with rounded edges and by surrounding a portion of the arc tube with quartz wool.
Another high efficiency, high intensity electrodeless discharge lamp is disclosed in U.S. Pat. No. 4,810,938 issued to P. D. Johnson, J. M. Anderson and J. T. Dakin on Mar. 7, 1989 and assigned to the assignee of the present invention. In this patent, which is hereby incorporated by reference, it is recognized that a particular combination of arc tube fill ingredients, including a sodium halide, a cerium halide and xenon, will achieve color rendition improvement without adversely affecting efficacy in these electrodeless HID lamps. Additionally, this patent presents a preferred structural configuration for further efficacy improvement.
Although such electrodeless metal halide HID lamps exhibit characteristics of high efficacy and good color, there are applications for which electroded lamp operation is advantageous. For example, at low power levels, i.e. below 200 watts, electroded HID lamps are generally more efficacious than electrodeless HID lamps. One reason is that small, low power, electrodeless HID lamps sustain high coil coupling losses. Moreover, the relative complexity and resulting high cost of the 13.56 MHz electronic ballasts generally needed to operate electrodeless HID lamps render such lamps impractical for many applications.
In another aspect of metal halide HID lamp operation, it is well-known that the discharge column of a lamp operated in the horizontal burning position is bowed, i.e., curved upwardly. Further, with regard to both horizontal and vertical operation, it is well-known that the discharge column may assume a contorted shape, which may be stable or unstable in the frequency range from approximately 20 kHz to approximately 1 MHz. Sufficiently severe instabilities can cause the discharge to extinguish. Moreover, an unstable arc may even destroy the arc tube. It has heretofore been generally accepted that these instabilities are related to the excitement of acoustic oscillations in the gas by the high frequency drive power. U.S. Pat. No. 4,170,746, issued to J. M. Davenport on Oct. 9, 1979 and assigned to the instant assignee, summarizes the theory and effects of destructive acoustic resonances. In particular, Davenport states that when commercially available metal halide HID lamps are operated in the range from 20 kHz to 50 kHz, they are subject to destructive effects of acoustics resonances. As a result, electronic ballast designs are generally limited to operating frequencies which do not excite acoustic resonant oscillations in the fill. In accordance with this theory, Davenport's invention involves miniature metal halide HID lamps operated in resonance-free regions.
In the 4th International Symposium on the Science and Technology of Light Sources, Apr. 7-10, 1986, p. 15, Alexander Dobrusskin describes advantages obtained by operating lamp ballasts at frequencies higher than 50 or 60 Hz, including increased efficacy and decreased arc bending. However, in accordance with the hereinabove described generally accepted theory of destructive acoustic resonances, he states that high pressure discharges are wasted or extinguish at frequencies which excite acoustic resonances in the plasma of the discharge. Further, he states that the difficulties in designing electronic ballasts involve selection of a suitable operating frequency and modulation of the supply voltage, i.e., elimination of supply voltage amplitude modulation.
Contrary to the above-described theory, acoustic resonant oscillations are excited in the fill of the HID lamp of the present invention by either an amplitude modulated or a non-amplitude-modulated power supply, to ensure thorough mixing of the ingredients comprising the fill, thereby resulting in high efficacy and straight arc operation even in the horizontal burning position.